Press -D-> button on GPS and there you go... ok now seriously.
I have never seen something like that and I can not imagine what you are dealing with but it is only PPL. With all respect.
Do you really have to plot it? It is PPL. I have map. I'm here and I want to go there that means turn around this tree, cross this river and over this village and it is done.
If you can not make it that simple then do not plot it from your possition. During our early navigation flights we had to cut our route into 5 minutes long parts. Do not start where you are. Start at next point so you will not be delayed or just hold while you plot it but I think it is much more difficult.
Could you be more specific how long flight it is, how high you fly? I'm really not familiar with PPL in UK but it is not IFR. You are looking out of window and follow what you see. I do not think it is ok not to fly. Even for a while.